Fissionable fuel grade uranium oxides for service in power generating nuclear reactors are commonly produced from uranium hexafluoride. There are two basic chemical procedures practiced in the industry for commercially carrying out this chemical conversion of uranium hexafluoride to uranium oxides for reactor fuel, and several variations on each.
One principal chemical conversion process is commonly referred to in this art as a "wet" process since the conversion reactions are affected by or carried out within an aqueous medium or liquid phase with the reactants in solution and/or as a solid suspension therein. Typically, this so-called wet process comprises hydrolyzing uranium hexafluoride (UF.sub.6) in liquid water to form the hydrolysis product uranyl fluoride (UO.sub.2 F.sub.2), adding ammonium hydroxide to the uranyl fluoride to precipitate the uranyl fluoride as solid ammonium diuranate [(NH.sub.4).sub.2 U.sub.2 O.sub.7 ], then dewatering the solids and calcining in a reducing atmosphere to produce an oxide of uranium (e.g. UO.sub.2). This version of the wet process is frequently referred to as the "ADU" procedure since it normally entails the formation of ammonium diuranate.
The other principal chemical conversion process is commonly referred to in this art as the "dry" process since the reactions are mainly carried out in a gaseous medium and comprise vapor-phase reactions with respect to the components. Typically, this so-called dry process comprises either a one-step procedure or a two-step procedure. The one-step method simply consists of contacting uranium hexafluoride gas (UF.sub.6) with a mixture of steam and hydrogen gas within a fluidized bed of particulate uranium oxide whereby solid uranium oxides (e.g. U.sub.3 O.sub.8) and hydrogen fluoride (HF) are produced. The U.sub.3 O.sub.8 is subsequently calcined in a reducing atmosphere to a lower oxide of uranium, e.g. UO.sub.2. The two-step method consists of hydrolyzing uranium hexafluoride gas (UF.sub.6) with steam to produce uranyl fluoride (UO.sub.2 F.sub.2) followed by reducing the uranyl fluoride with both steam and hydrogen gas to an oxide of uranium (e.g. UO.sub.2).
The uranium oxides commercially produced by such conventional methods comprise a fine relatively porous powder which is not suitable as such for use as fuel in a nuclear reactor. Typically, it is not a free-flowing, relatively uniform sized powder, but rather clumps and agglomerates of particles of varying sizes making it unsuitable to uniformly pack into units of an apt and consistent density These uranium oxide powders often have very high particle surface areas.
Thus, the raw uranium oxide product derived from the chemical conversion is normally processed through conventional powder refining procedures such as milling and particle classification to provide an appropriate sizing of the powders. Such processing frequently includes blending of uranium oxide powders of different particle sizes or ranges and from different sources. Commonly the powdered uranium oxides are handled and conveyed through such processing operations by pneumatic means. Thus, the uranium oxides can be subjected to extensive exposure to air, and in turn, oxygen.
Aptly processed uranium oxide powders are press molded into "green" or unfired pellets which are subsequently sintered to fuse the discrete powder particles thereof into an integrated body having a unit density of about 98 percent of theoretical for the oxide of uranium, and suitable for utilization in the fuel system of a nuclear reactor.
Uranium dioxide is an exception to the law of definite proportions since "UO.sub.2 " actually denotes a single, stable phase that may vary in composition from UO.sub.1.7 to UO.sub.2.25. The thermal conductivity of uranium oxide decreases with increasing oxygen to uranium ratios. Thus, uranium dioxide having as low an O/U ratio as practical is preferred for use as fuel in nuclear reactors to enable the most efficient passage of heat generated within fissioning fuel material outward to an external heat transfer medium. However, since uranium dioxide powder oxidizes readily in air and absorbs moisture, the oxygen to uranium (O/U) ratio of the powder will increase significantly to an excess of that acceptable for use as nuclear fuel for effective operation of a nuclear reactor.
Uranium oxides suitable for fuel in typical nuclear reactor service can have an O/U ranging from about 1.70-2.015 to 1, and as a practical matter, an O/U ratio of approximately 2.0 and effectively as high as 2.015 has been used since it can be consistently produced in commercial sintering operations. In some instances, it may be practical to maintain the O/U ratio of the uranium dioxide at a level higher than about 2.00 at sintering temperature. For example, it may be more suitable under the particular manufacturing process to produce a nuclear fuel having an O/U ratio as high as 2.195, and then later treat the sintered product in a reducing atmosphere to obtain the desire U/O ratio. However, such an extra operation usually increases costs without added benefits.
Uranium oxides of low O/U ratios exhibit an especially high propensity for spontaneous oxidation to a higher ratio, or actual burning in air, which can be hazardous as well as introducing deleterious properties in the uranium oxides for their intended service. The magnitude of this affinity for oxidation of uranium and the rate of the oxidizing reaction is influenced by a number of conditions, in particular ambient temperatures, oxygen partial pressure and surface area of the oxide of uranium particles. Moreover, since the oxidation reaction of uranium is exothermic, the oxidation of uranium and the rate thereof is self-propagating and accelerating.
This proclivity to further oxidation of uranium oxides presents a significant factor, or potential problem when particulate oxides of uranium are stored or undergo processing such as milling, classifying or blending in unprotected atmospheres not excluding oxygen.
The presence of a high oxygen content in uranium oxides is known to modify its behavior in both processing or fabrication, and in performance of nuclear fuel. For instance, an oxygen ratio in excess of the uranium dioxide U/O.sub.2 stoichiometric ratio has a decided effect upon the sintering rate of uranium-oxides by accelerating its completion, and/or enabling temperature reduction.
On the other hand, a high oxygen content in uranium oxide utilized as nuclear fuel in a reactor is generally overall detrimental A high oxygen ratio in uranium oxide nuclear fuel reduces the thermal conductivity of the fuel mass, increases the diametral expansion of the body of fuel under irradiation conditions during operation within a nuclear reactor, and the ratio of fission products released from the uranium oxide fuel is increased, among other effects.
The influence of oxygen content upon uranium oxide nuclear fuel is considered in detail in an article entitled "The Storage Behavior of Uranium Dioxide Powders" by M. J. Bannister, Journal of Nuclear Materials, 26 (1968), pages 174-184.